This invention relates to a gas-permeable porous film provided with a pattern formed by disposing regularly elevations and depressions (this pattern is hereinafter referred to as a rugged pattern) on its surface and a preparation process thereof.
Conventionally, it has been known to prepare a porous film by blending inorganic fine powder with a polyolefin resin in specific proportions, melting and forming the resulting composition into a film, and stretching the film at least uniaxially (Japanese Patent Publication No. 12542/1978 and Japanese Patent Laid-Open Nos. 99242/1981 and 59727/1982).
Although the porous films obtained by these processes had for their function a gas permeability, they failed to appeal their moisture-permeability to the visual sense and thus lacked an appeal to consumers of their gas-permeability. They were devoid of a depth in appearance and hence looked cheapish and strength-lacking. In addition, the films involved such drawbacks that upon touching them, the skin felt a coldness specific to the films made of resins and in some cases a surface tackiness.
With the aim of improving these processes, it has been proposed to subject a film containing an inorganic filler simply to an embossing treatment (Japanese Patent Laid-Open Nos. 30856/1976 and 80450/1985). The process disclosed in Japanese Patent Laid-Open No. 30856/1976 has a disadvantage that when a film is embossed to such an extent that a sufficient gas permeability is imparted thereto, the film may break and inevitably have large holes partially evolved. Further, in the process of Japanese Patent Laid-Open No. 80450/1985, no patterns with satisfactory elevations and depressions (elevations and depressions are hereinafter referred to as ruggedness) are formed on the film.
The porosity of a porous film containing a filler is generally dependent on the amount of the filler used, its particle size, stretching ratio and similar factors. Pores are formed by stretching a film at a stretching ratio of 2.0 times or more in one direction. However, in order to attain a sufficient porosity through uniform pore distribution, it is necessary to use a stretching ratio as high as 4 times or more. Accordingly, the porous films obtained by the conventional processes have disadvantages in mechanical strength, for example, low tear strength in the stretching direction for those obtained by uniaxial stretching and weakness in surface strength for those from biaxial stretching.
The porous films obtained at a low stretching ratio have sufficient tear strength. However, at a low stretching ratio, uniform stretching is hardly brought about and stretching irregularities are liable to occur, with the result that the films thus-formed are extremely poor-looking and have degraded values as a product. In addition, gas-permeability of the resulting porous films is so uneven that the films are regarded as unfavorable from a functional viewpoint.